1. Field of the Invention
The invention concerns a device for connecting a data communication network to a plurality of subscribers, a subscriber being in the present context a sender and/or a receiver of data.
2. Description of the Prior Art
Known data communication buses can convey data, for example digital data, over one or more conductors between sensors and actuators (valves, motors, pumps, etc) and control units such as programmable automatic controllers, microprocessors and dedicated computers.
Communication buses of this kind are used, for example, to provide communication between units of this type used for home management (home automation application). The connection of the various units to a communication bus common to all units is an advantageous substitute for point-to-point wiring (star network) using the de facto 40-20 mA standard. The cost of wiring is reduced because the lengths of the connecting wires are reduced, electromagnetic interference is virtually eliminated and installation is greatly facilitated.
This type of network is also used in field networks for automating product manufacturing units. A unit (sensor, actuator, computer ruggedized for use in an industrial environment, etc) is connected to the data communication bus (or main cable or TRUNK) as shown in FIG. 1.
FIG. 1 shows a field network comprising a plurality of subscribers.
A field network 10, of the Factory Instrumentation Protocol (FIP) type, for example, comprises a main cable conveying data serially over two twisted electrical conductors. The data is conveyed between subscribers CAC1, CAC2, . . . , CAC30 each consisting of a sensor, an actuator or a computer. The connection between these subscribers is made by means of drop and insert devices. A device of this kind is often called a TAP.
Both ends of the TRUNK link comprises means LT for matching the transmission line between each of the end of line TAPs TAP1 and TAP30.
The drop and insert connections made by the TAPs involve cutting the TRUNK link, inserting a TAP at the break and connecting the two sides of the link to each subscriber CAC1 through CAC30 as shown in FIG. 1A. This prevents the mismatching of the line that would occur in the case of a simple drop and insert connection from the TRUNK link to a subscriber.
The connections between each TAP and each subscriber are provided by identical cables 11 whose end opposite the TAP is fitted with a connector 12. The connector 12 shown in FIG. 1A comprises two connecting terminals 13 to which the wires from the TAP are connected in pairs to maintain the continuity of the electrical circuit. The shielding wires are not shown.
The dashed lines 14 in FIG. 1 represent the electromagnetic shielding of the subscribers connected to the shields 15 of the cables 11.
The network 10 therefore comprises the same number of TAPs and subscribers CAC.
The main drawback of this type of network is that when any new subscriber is added, for example to connect sensors and actuators of a new machine to the TRUNK line, it is necessary to insert a new TAP into the TRUNK line. The TAPs are drop and insert modules which are costly because they must meet specific standards, especially with regard to ruggedness and safety. In some cases the TRUNK links are buried and it is then necessary to excavate them to insert one or more TAPs, introducing considerable constraints in addition to the time needed to rebury the TRUNK link.
Also, because of the line mismatch that would result, it is not possible to connect in parallel with an existing cable another cable to a subscriber to be added to an existing installation.
The abstract of Japanese patent No 58.212245 in the name of FUJI XEROX K.K. describes a device for connecting main bus subscribers in the form of a drop and insert connection to the bus. Subscribers are connected in series to the drop and insert point by means of connectors. This solution has the drawback previously explained, i.e. that connecting this connecting device to the main bus produces a mismatch (the subscribers are connected in series to the bus). The impedance matching problem is not treated in this document because each subscriber can have its own input impedance and all the subscribers must be connected to the connecting device to ensure continuity.
A particular object of the present invention is to overcome these drawbacks.
To be more precise, one object of the invention is to provide a connecting device for field buses enabling at least two subscribers to be added to the bus quickly and easily without causing any mismatching of the bus.
Another object of the invention is to provide a connecting device of this kind which is less costly and easier to manufacture than those used in the prior art.